krcrisp wrote:After the WZ68 failed and blew the fuse, I looked around on this forum and saw the post from the moderator along the lines of "if you want to try ss rectification, do this...." and I did it as suggested, then adjusted the bias. I assumed Bob wouldn't suggest it if it was a bad idea. I have the time delay installed; it takes 19 seconds to click on.
My line voltage is 119V as of today. I do have a variac. The copper cap that Webber recommends for hifi is just diodes. I do have a new WZ68 I could put back in. The 18 month life on the prior one did not impress me.
OK, and please excuse my ignorance, but how do I test the plate voltage and what should it be? I assume a pin of a power tube to chassis but I don't know which one.
As I'm using a pair of Cornwalls I am really not using the power of the amp much.
If Bob recommends it I defer to his wisdom. My assumption was that the ST120 is an old Dynaco design (not to be confused with the Dynaco Stereo 120, which was a solid state), based on tube rectifiers.
You built it, so I assume you are comfortable measuring potentially lethal voltages, right? But the manual that came with it should specify normal voltages.
One thing that should be easy to do is measure the voltages to ground on all the terminals of the filter caps, and compare that to the working voltage marked on the caps. Do you have antique replica multi-section can capacitors? The "Authenticaps" in my M125's did not at all appreciate being run without the power tubes plugged in, to test the power supplies (contrary to Bob's instructions) and I ended up frying one.
Weber recommends the W68 for guitar amps, where the amp is part of the instrument and the rectifier voltage drop contributes to the sound, and the one without dropping resistors for hi fi, because hi fi users presumably prefer an uncolored sound (and I am definitely in that camp). But in making the substitution you have to make sure your amp can take the inevitably higher voltage that results.